The cytoplasmic matrix is a complex meshwork of filaments and microtubules tied together by innumerable filamentous bridges. This structure and the water-filled spaces give to the cytoplasm its gel-like consistency. Ordinarily, in animal cells, the cytomatrix, with inclusions, is organized radially around a center comprising two centrioles and various arrangements of associated dense bodies. The latter function as initiating sites for the assembly and disposition of microtubules. These are skeletal in function and by physical association with the matrix they inflict form asymmetries on the cell and function as guides for the intracellular translocation of cytoplasmic granules, vesicles, etc. From studies of these structures and their behaviour there has come the concept that the cytoplasm is a unit structure or cytoplast in which functional and structural components occupy fixed positions. The studies proposed in this grant request aim to the characterize the relationships between matrix and microtubules. Chromatophores, pigment colored cells, from fish are used, partly because the pigment can be followed easily in the living cell by light microscopy and partly because the fully differentiated cells translocate pigment rapidly along tracks of microtubules. By severing the arms of melanophores it is proposed to generate minicells which contain pigment, or not, and populations of microtubules. They can be kept for days under in vitro conditions. Since they have no centrioles their microtubules are assembled and oriented by factors within the cytomatrix. It is hoped that some knowledge of these factors will be acquired. At the same time, it is proposed to explore the properties of cytoplasts by fusing them into other types of cells such as NRK's and BSC's. Do the cytoplasts of red pigment cells retain or lose their identity in the cytoplasms of larger NRK cells? Actually they don't; rather they dominate the NRK's and sequester the tubulin of the rat cell for their own use. It is expected that further exploration of this and other phenomena will provide further insight into cytoplasmic organization, how it is achieved and maintained.